Osteosarcoma is the most common primary bone cancer, and the third most common solid tumor in adolescents, affecting nearly 1000 people per year in the United States. This disease desperately needs new therapies, since nearly 40% of those diagnosed will die of their disease, and the survival for this disease has not truly improved in more than 25 years. Signals from the ERBB family of receptor tyrosine kinases contribute essential signals to osteosarcoma, and that a pan-ERBB inhibitor is more effective at stopping the growth of osteosarcoma than is a selective EGFR inhibitor or an inhibitor that blocks signals from either EGFR or Her-2. Most osteosarcoma is do express EGFR, and that those which express Her-2 have a greater propensity to metastasize. However, the specific contributions of Her-2 and Her-4 to malignant behavior in osteosarcoma are largely undocumented. Of the four ERBB family members, Her-4 is the least well characterized. Based upon the in vitro superiority of pan ERBB inhibition compared to inhibition of EGFR and Her-2 alone, it is hypothesized that Her-4 makes unique contributions to osteosarcoma pathology. Likewise, since Her-2 is associated with a more metastatic phenotype, it is likely that it also has unique contributions. This proposal is intended to evaluate the scientific basis for therapies targeting the ERBB family of receptor tyrosine kinases for treating osteosarcoma, by defining the unique contributions of each of the family members within this disease. In the first specific aim, molecular approaches are used to evaluate the precise contributions of each ERBB family member to osteosarcoma biology, focusing particularly on the role of Her-4 in the nucleus and its potential to regulate the expression of other genes. In the second specific aim, a large bank of archival osteosarcoma specimens is evaluated for expression of members of the ERBB family, correlating expression with clinical outcome. In the third specific aim, to novel orthodontic osteosarcoma xenograft models are used to evaluate the therapeutic potential of a specific pan ERBB inhibitor, PF00299804. The effect of eliminating individual ERBB family members also is evaluated. When successfully completed, the proposed studies will define the specific contributions of each ERBB family member to osteosarcoma biology, understanding their role in a clinical context, and potentially providing the basis for using a pan ERBB inhibitor for treating patients with osteosarcoma. Should the underlying hypothesis be proven, these studies would pave the way for new clinical trials for children who desperately need new treatments.